Use -X for language extensions

Use -X for language extensions

We've often talked about having a separate flag for language extensions,
and now we have one. You can say

	-XImplicitParams
	-X=ImplicitParams
	-Ximplicit-params

as you like.  These replace the "-f" flags with similar names (though
the -f prefix will serve as a synonym for -X for a while).

There's an optional "=", and the flag is normalised by removing hyphens
and lower-casing, so all the above variants mean the same thing.

The nomenclature is intended to match the LANGUAGE pramgas, which are
defined by Cabal.  So you can also say

	{-# LANGUAGE ImplicitParams #-}

But Cabal doesn't have as many language options as GHC does, so the -X
things are a superset of the LANGUAGE things.

The optional "=" applies to all flags that take an argument, so you can,
for example, say

	-pgmL=/etc/foo

I hope that's ok.  (It's an unforced change; just fitted in.)

I hope we'll add more -X flags, to replace the portmanteau -fglasgow-exts
which does everything!

I have updated the manual, but doubtless missed something.